Such a particle source is known from U.S. Pat. No. 6,770,878, describing a charged particle source with an integrated omega filter. The charged particle source comprises an ion or electron emitter, such as a Schottky emitter, emitting charged particles in a cone along an axis. A beam defining aperture selects an axial part of the beam, and focusing electrodes image the source further on along the axis. A first hemispherical capacitor acting as a deflector bends the axis, followed by a second hemispherical capacitor bending the axis parallel to its original direction, but displaced. A third and a fourth hemispherical capacitor bend the axis back so that it is both parallel and aligned with its original direction, that is: before it entered the first deflector. The axis of the filter thus resembles the Greek letter Ω, hence its name.
Between the second and the third deflector the earlier mentioned focusing electrodes form an astigmatic image of the source. Due to the deflection of the axis the image shows energy dispersion. An energy selecting aperture or slit is placed in the image plane, and a part of the beam with a desired energy spread is selected from the beam and transmitted, while electrons with an energy spread outside the desired energy spread are intercepted.
It is thus clear that the mechanical symmetry axis of the electrode surfaces follow the optical axis of the filter.
It is noted that the known source is made to operate in one of two ways: a first setting in which energy filtering is achieved by bending the axis and thus the beam with the hemispherical capacitors so that an image is formed on the energy selecting aperture, and a second setting in which the four hemispherical capacitors are not excited, and the axis extends straight from the charged particle emitted to the output of the source, through small holes in the electrodes of the first and fourth capacitor. In this second setting no energy selection takes place.
It is a disadvantage of the known source that the hemispherical capacitors are off-axial, and both machining and aligning the mechanical parts forming these hemispherical capacitors is complicated.
A generalized version of the filter discussed in U.S. Pat. No. 6,770,878 is given in U.S. Pat. App. No. 2008/0290273 A1. Here not only the Ω filter, but also the a filter is discussed, as are the optics needed for the elimination of astigmatism. According to this application the skilled person (the expert) must first select a shape for the axis of the monochromator, e.g. an Ω or an α, resulting in a similar disadvantage as described for the U.S. Pat. No. 6,770,878.
It is an object of the invention to provide a remedy for these disadvantages.